1. Field of the Invention
The present invention concerns a solid state p-n heterojunction, comprising an electron conductor, i.e. an n-type semiconductor, and a hole conductor, i.e. an p-type semiconductor. The invention concerns also a solid state sensitized photovoltaic cell, in particular a solar cell.
2. Description of Related Art
The dye sensitized solar cells (DSSC), are more and more maturing into a technically and economically credible alternative to the conventional p-n junction photovoltaics. Photoelectrochemical (PEC) liquid junction cells based on the photosensitization of nanocrystalline TiO2 semiconductor layers with molecular sensitizers attracted renewed interest after Grätzel et al reported energy conversion efficiencies >10%, in Nature, 353 (1991) 737. The nanocrystalline semiconductor network forming a mesoporous structure, characterized by its high surface area, has several design functions: it provides the surface for the sensitizer adsorption, it accepts the electrons from the light-excited dye and finally it conducts the injected electrons to the working electrode.
In 1998, Grätzel et al reported in Nature, 395 (1998) 583, a more innovative all solid state cell based on a heterojunction sensitized by a molecular dye where an amorphous organic hole transport material replaced the liquid electrolyte.
The crucial part in these cells is the dye itself. Only a very limited number of dyes give high photocurrent quantum yields and are reasonably stable against photo-degradation. Some of the organic dyes exhibiting high light absorption are sensitive to air and water.
Siebentritt et al (14th European Photovoltaic Solar Energy Conference, 1997) proposed to provide porous TiO2 layers with thin films of a light absorber material, e.g. CdS films, or CdTe films with thicknesses of about 100 nm. The film forming material is deposited either by electrodeposition or by chemical bath deposition. Depending upon the type of TiO2 material (spray-pyrolysed or screen-printed), the absorber material film grows substantially on top of the TiO2 layer, where no external limitation to the particle size exists, or penetrates into the structure, filling up the pores. According to the teaching of Siebentritt et al, an annealing step, increasing the size of crystallites within the films, improves the photoresponse. But one has to consider though that this teaching represents only “half” a solar cell with no p-type semiconductor being present.